It is well known that oscillators and/or clocks will, over time, produce outputs having frequency and/or time accuracy errors.
Any two independent clocks, for example, once synchronized, will walk away from one another without bound, and the difference between them will exceed any limit given enough elapsed time. Improvements in the performance of a clock unit may slow this process but will not eliminate it.
One method for maintaining agreement between two clock (or oscillator) units involves a continuous, or nearly continuous, communication between them. This is inconvenient and impractical in general, and most clocks, for example, are periodically reset (using correct time from an external reference) to maintain synchronism. Although the two clock units agree each time the reset is performed, the clock readings walk away from one another between resets.
Some systems attempt to correct for clock error, for example, by using an estimate of the clock rate difference. None of the heretofore known methods of either periodic clock reset or the use of static rate offset, however, optimally utilize the information available to construct a statistically robust model of the performance of the actual clock in its environment and they are therefore much less efficient than they could be.
Errors due to time offset, frequency offset, frequency drift, clock noise and external perturbations all contribute to inaccuracies of an oscillator/clock unit output between calibrations, or updates, and existing processes provide no means for assessing the accuracy of output readings during these intervals.